This invention relates to a circuit arrangement for the compensation of a control current of a first transistor whose main current path is arranged in series with a main current path of a second transistor between two supply voltage terminals, the arrangement comprising a current mirror circuit having at least two transistors and having a common terminal connected to the supply voltage terminal, which is coupled to the second transistor, an input terminal of the current mirror being connected to a control terminal of the second transistor and an output terminal of the current mirror being arranged to supply a compensation current to a control terminal of the first transistor.
From DE-AS 21 08 550 which corresponds to U.S. Pat. No. 3,714,600 (Jan. 30, 1973), a transistor amplifier is known which comprises a first transistor to whose base a signal to be amplified is applied. The emitter of the first transistor is connected to a point of constant potential, preferably to ground, via a resistor. The collector of the transistor is connected to the emitter of a measurement transistor. The collector of the measurement transistor is connected to a supply voltage source via a second resistor. The base of the measurement transistor is connected to a current input of a controlled current source and the base of the first transistor is connected to a current output of the controlled current source. A common terminal of the current source is connected to the collector of the measurement transistor. The current source comprises a transistor and a diode. The emitter of the transistor of the current source and the anode of the diode are connected to the common terminal of the current source. The collector of the transistor of the current source is connected to the current output of this source and the base of the transistor of the current source and the cathode of the diode are connected to the current input of the current source.
By means of the current source in this circuit arrangement, it is possible to reduce the input current of the amplifier by a factor which substantially corresponds to the base-collector current gain factor of the first transistor and the measurement transistor.
From U.S. Pat. No. 3,916,331 it is further known to replace the current source described above by a current source which is generally also referred to as a "Wilson current mirror". This "Wilson current mirror" comprises a third and a fourth transistor and a diode. The emitter of the third transistor and the anode of the diode are connected to the above-mentioned common terminal of the current source. The base of the third transistor and the cathode of the diode are connected to the emitter of the fourth transistor, whose base is connected to the collector of the third transistor. The collector of the fourth transistor is connected to the output of the "Wilson current mirror" and the collector of the third transistor is connected to the input of the "Wilson current mirror". In comparison with the current source described above, the use of the "Wilson current mirror" has the advantage that the currents appearing at the output and at the input of the "Wilson current mirror" are in better correspondence to one another. This provides an improved compensation of the base current of the first transistor.
However, in comparison with the first-mentioned circuit arrangement the last-mentioned arrangement has the drawback that as a result of the use of the "Wilson current mirror" the potential on the collector of the first transistor is reduced by one further base-emitter forward voltage relative to the potential on the collector of the measurement transistor and on the common terminal of the current source. The range of a signal to be amplified by the transistor amplifier comprising the first transistor is then reduced by this amount. This constitutes a considerable limitation, in particular if the supply voltage for the transistor amplifier should be very small because the further base-emitter forward voltage forms a considerable part of this supply voltage.